Re: [PATCH 2/3] mm/swap: use swap_ops to register swap device's methods

[Barry Song <21cnbao@gmail.com>]

On Mon, Mar 2, 2026 at 6:40 PM Baoquan He <bhe@redhat.com> wrote:
>
> This simplifies codes and makes logic clearer. And also makes later any
> new swap device type being added easier to handle.
>
> Currently there are three types of swap devices: bdev_fs, bdev_sync
> and bdev_async, and only operations read_folio and write_folio are
> included. In the future, there could be more swap device types added
> and more appropriate opeations adapted into swap_ops.
>
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  include/linux/swap.h |  13 ++++++
>  mm/swap.h            |   1 -
>  mm/swap_io.c         | 102 +++++++++++++++++++++++++------------------
>  mm/swapfile.c        |   2 +
>  mm/zswap.c           |   3 +-
>  5 files changed, 76 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 0effe3cc50f5..448e5e66ec5c 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -19,6 +19,7 @@
>  struct notifier_block;
>
>  struct bio;
> +struct swap_iocb;
>
>  struct pagevec;
>
> @@ -222,6 +223,17 @@ enum {
>  #define SWAP_CLUSTER_MAX_SKIPPED (SWAP_CLUSTER_MAX << 10)
>  #define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX
>
> +struct swap_ops {
> +       void (*read_folio)(struct swap_info_struct *sis,
> +                          struct folio *folio,
> +                          struct swap_iocb **plug);
> +       void (*write_folio)(struct swap_info_struct *sis,
> +                           struct folio *folio,
> +                           struct swap_iocb **plug);
> +};
> +
> +int probe_swap_fs(struct swap_info_struct *sis);

Does probe_swap_fs sound a bit odd?
What about init_swap_ops? Not sure if we have a better name.

Do we really want it, along with swap_ops, to live in
include/linux/swap.h? Could it be placed in mm/swap.h instead?

> +
>  /*
>   * The first page in the swap file is the swap header, which is always marked
>   * bad to prevent it from being allocated as an entry. This also prevents the
> @@ -284,6 +296,7 @@ struct swap_info_struct {
>         struct work_struct reclaim_work; /* reclaim worker */
>         struct list_head discard_clusters; /* discard clusters list */
>         struct plist_node avail_list;   /* entry in swap_avail_head */
> +       struct swap_ops *ops;
>  };
>
>  static inline swp_entry_t page_swap_entry(struct page *page)
> diff --git a/mm/swap.h b/mm/swap.h
> index 161185057993..c390df3f5889 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -226,7 +226,6 @@ static inline void swap_read_unplug(struct swap_iocb *plug)
>  }
>  void swap_write_unplug(struct swap_iocb *sio);
>  int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
> -void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
>
>  /* linux/mm/swap_state.c */
>  extern struct address_space swap_space __read_mostly;
> diff --git a/mm/swap_io.c b/mm/swap_io.c
> index d1cdb10ba133..47077b345ae3 100644
> --- a/mm/swap_io.c
> +++ b/mm/swap_io.c
> @@ -240,6 +240,7 @@ static void swap_zeromap_folio_clear(struct folio *folio)
>  int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
>  {
>         int ret = 0;
> +       struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
>
>         if (folio_free_swap(folio))
>                 goto out_unlock;
> @@ -281,7 +282,8 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug)
>                 return AOP_WRITEPAGE_ACTIVATE;
>         }
>
> -       __swap_writepage(folio, swap_plug);
> +       if (sis->ops && sis->ops->write_folio)
> +               sis->ops->write_folio(sis, folio, swap_plug);

Do we want a swap_write_folio() wrapper?

>         return 0;
>  out_unlock:
>         folio_unlock(folio);
> @@ -371,10 +373,11 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
>         mempool_free(sio, sio_pool);
>  }
>
> -static void swap_writepage_fs(struct folio *folio, struct swap_iocb **swap_plug)
> +static void swap_writepage_fs(struct swap_info_struct *sis,
> +                             struct folio *folio,
> +                             struct swap_iocb **swap_plug)
>  {
>         struct swap_iocb *sio = swap_plug ? *swap_plug : NULL;
> -       struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
>         struct file *swap_file = sis->swap_file;
>         loff_t pos = swap_dev_pos(folio->swap);
>
> @@ -407,8 +410,9 @@ static void swap_writepage_fs(struct folio *folio, struct swap_iocb **swap_plug)
>                 *swap_plug = sio;
>  }
>
> -static void swap_writepage_bdev_sync(struct folio *folio,
> -               struct swap_info_struct *sis)
> +static void swap_writepage_bdev_sync(struct swap_info_struct *sis,
> +                                    struct folio *folio,
> +                                    struct swap_iocb **plug)
>  {
>         struct bio_vec bv;
>         struct bio bio;
> @@ -427,8 +431,9 @@ static void swap_writepage_bdev_sync(struct folio *folio,
>         __end_swap_bio_write(&bio);
>  }
>
> -static void swap_writepage_bdev_async(struct folio *folio,
> -               struct swap_info_struct *sis)
> +static void swap_writepage_bdev_async(struct swap_info_struct *sis,
> +                                     struct folio *folio,
> +                                     struct swap_iocb **plug)
>  {
>         struct bio *bio;
>
> @@ -444,29 +449,6 @@ static void swap_writepage_bdev_async(struct folio *folio,
>         submit_bio(bio);
>  }
>
> -void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug)
> -{
> -       struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
> -
> -       VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);
> -       /*
> -        * ->flags can be updated non-atomically (scan_swap_map_slots),
> -        * but that will never affect SWP_FS_OPS, so the data_race
> -        * is safe.
> -        */
> -       if (data_race(sis->flags & SWP_FS_OPS))
> -               swap_writepage_fs(folio, swap_plug);
> -       /*
> -        * ->flags can be updated non-atomically (scan_swap_map_slots),
> -        * but that will never affect SWP_SYNCHRONOUS_IO, so the data_race
> -        * is safe.
> -        */
> -       else if (data_race(sis->flags & SWP_SYNCHRONOUS_IO))
> -               swap_writepage_bdev_sync(folio, sis);
> -       else
> -               swap_writepage_bdev_async(folio, sis);
> -}
> -
>  void swap_write_unplug(struct swap_iocb *sio)
>  {
>         struct iov_iter from;
> @@ -535,9 +517,10 @@ static bool swap_read_folio_zeromap(struct folio *folio)
>         return true;
>  }
>
> -static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
> +static void swap_read_folio_fs(struct swap_info_struct *sis,
> +                              struct folio *folio,
> +                              struct swap_iocb **plug)
>  {
> -       struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
>         struct swap_iocb *sio = NULL;
>         loff_t pos = swap_dev_pos(folio->swap);
>
> @@ -569,8 +552,9 @@ static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
>                 *plug = sio;
>  }
>
> -static void swap_read_folio_bdev_sync(struct folio *folio,
> -               struct swap_info_struct *sis)
> +static void swap_read_folio_bdev_sync(struct swap_info_struct *sis,
> +                                     struct folio *folio,
> +                                     struct swap_iocb **plug)
>  {
>         struct bio_vec bv;
>         struct bio bio;
> @@ -591,8 +575,9 @@ static void swap_read_folio_bdev_sync(struct folio *folio,
>         put_task_struct(current);
>  }
>
> -static void swap_read_folio_bdev_async(struct folio *folio,
> -               struct swap_info_struct *sis)
> +static void swap_read_folio_bdev_async(struct swap_info_struct *sis,
> +                                      struct folio *folio,
> +                                      struct swap_iocb **plug)
>  {
>         struct bio *bio;
>
> @@ -606,6 +591,42 @@ static void swap_read_folio_bdev_async(struct folio *folio,
>         submit_bio(bio);
>  }
>
> +static struct swap_ops bdev_fs_swap_ops = {
> +       .read_folio = swap_read_folio_fs,
> +       .write_folio = swap_writepage_fs,
> +};

const?

> +
> +static struct swap_ops bdev_sync_swap_ops = {
> +       .read_folio = swap_read_folio_bdev_sync,
> +       .write_folio = swap_writepage_bdev_sync,
> +};

const?

> +
> +static struct swap_ops bdev_async_swap_ops = {
> +       .read_folio = swap_read_folio_bdev_async,
> +       .write_folio = swap_writepage_bdev_async,
> +};

const?

> +
> +int probe_swap_fs(struct swap_info_struct *sis)
> +{
> +       /*
> +        * ->flags can be updated non-atomically (scan_swap_map_slots),
> +        * but that will never affect SWP_FS_OPS, so the data_race
> +        * is safe.
> +        */
> +       if (data_race(sis->flags & SWP_FS_OPS))
> +               sis->ops = &bdev_fs_swap_ops;
> +       /*
> +        * ->flags can be updated non-atomically (scan_swap_map_slots),
> +        * but that will never affect SWP_SYNCHRONOUS_IO, so the data_race
> +        * is safe.
> +        */
> +       else if (data_race(sis->flags & SWP_SYNCHRONOUS_IO))
> +               sis->ops = &bdev_sync_swap_ops;
> +       else
> +               sis->ops = &bdev_async_swap_ops;
> +       return 0;
> +}
> +
>  void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
>  {
>         struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
> @@ -640,13 +661,8 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
>         /* We have to read from slower devices. Increase zswap protection. */
>         zswap_folio_swapin(folio);
>
> -       if (data_race(sis->flags & SWP_FS_OPS)) {
> -               swap_read_folio_fs(folio, plug);
> -       } else if (synchronous) {
> -               swap_read_folio_bdev_sync(folio, sis);
> -       } else {
> -               swap_read_folio_bdev_async(folio, sis);
> -       }
> +       if (sis->ops && sis->ops->read_folio)
> +               sis->ops->read_folio(sis, folio, plug);
>
>  finish:
>         if (workingset) {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 915bc93964db..af498f9af328 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3625,6 +3625,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>         /* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */
>         enable_swap_info(si);
>
> +       probe_swap_fs(si);

Can we move this to enable_swap_info(), or perhaps even
deeper into setup_swap_info()?

> +
>         pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s\n",
>                 K(si->pages), name->name, si->prio, nr_extents,
>                 K((unsigned long long)span),
> diff --git a/mm/zswap.c b/mm/zswap.c
> index a399f7a10830..7ce906249c7a 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1055,7 +1055,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>         folio_set_reclaim(folio);
>
>         /* start writeback */
> -       __swap_writepage(folio, NULL);
> +       if (si->ops && si->ops->write_folio)
> +               si->ops->write_folio(si, folio, NULL);

swap_write_folio() inline wrapper?

>
>  out:
>         if (ret && ret != -EEXIST) {
> --
> 2.52.0
>

Thanks
Barry